To meet the 1000 times growth of wireless traffic in the next decade, the millimeter wave transmission technology is becoming one of the key technologies for the future fifth generation (5G) mobile networks [1]}. With the implementation of millimeter wave transmission technologies, a lot of frequency bandwidths are utilized for 5G mobile communication systems [2]}. Moreover, the high spectrums, e.g., 100-300 GHz spectrums are proposed for future mobile networks [3]}, [4]}. Consequently, the orbital angular momentum (OAM) technology traditionally used in optical communications is explored to be used for millimeter wave communications in the high spectrums [5]}. Compared with the conventional degree of freedoms in wireless communications, such as degree of freedoms in time, frequency and space, the OAM technology provides a new degree of freedom, i.e., the OAM state for millimeter wave communications [6]}. A revolutionary improvement on millimeter wave communications can be expected for future mobile networks when the OAM technology is adopted for millimeter wave communications.
